1. Field of the Invention
The present invention relates generally to calibration of electronic devices. More particularly, the present invention relates to calibration of amplifiers by modifying the transfer function associated with a variable transfer function, and most usefully to high voltage difference amplifiers with unmatched external input resistors. Embodiments of the invention include circuitry which periodically or continuously auto-calibrates an electronic device by adjusting variable transfer function circuitry of the electronic device while the electronic device is receiving an operating signal.
2. State of the Art
Signal quality is a common concern among designers and manufacturers of precision electronic signal conditioning devices. Unfortunately, however, variations in production processes, environmental variations, such as temperature, package assembly stress, and the like, and variations caused by differences in component aging, can make it difficult to achieve high signal conditioning accuracy. Difference amplifiers, for example, are designed to cancel a common-mode signal at their inputs and pass only the differential signal between the two input signal lines. The ability to cancel the common-mode input signal is called common-mode rejection. A precision difference amplifier can be made with an op amp and four precision resistors. Its ability to cancel the common-mode signal depends on resistor matching. When certain resistors have insufficient matching accuracy, signals which are common to both input signal lines may appear to be differential signals. Components which are selected with very tight tolerances, sufficient to provide high common-mode rejection, can be relatively expensive compared to standard components.
To overcome the need to use expensive specialized components in electronic devices, various calibration procedures are commonly used to compensate for component variations. Conventional calibration can be performed in a number of different ways and at various times during the manufacture or use of a product. Typically, calibration may be performed at a production factory prior to incorporating the electronic device into larger electronic systems. Factory calibration, however, can be expensive and, without periodic or semi-continuous calibration, the electronic devices are subject to drift due to environmental and other factors. For periodic or semi-continuous calibration, there is a need for input switches and control circuitry to isolate the input signal during the calibration adjustment phase. Examples of calibration procedures using isolation switches are shown and described in U.S. Pat. No. 4,086,541 to Katou et al. (Apr. 25, 1978) and U.S. Pat. No. 5,424,677 to Carson (Jun. 13, 1995). The switches required to isolate high input voltages in particular, such as those required for high voltage difference amplifiers, can be particularly expensive to implement. Traditional low voltage integrated circuit processes do not include high voltage switches. Circuits built on high voltage processes typically cost more than that of comparable circuits built on low-voltage processes due to the larger size required for high voltage components.
Calibration is conventionally incorporated into an initialization phase of an electronic device such as that disclosed in U.S. Pat. No. 6,069,533 to Kim (May 30, 2000). Kim discloses providing a DC signal of a predetermined value to two input terminals of an electronic circuit for an audio/video system during the initial ‘power-on’ sequence of the system. A feedback loop from the output of a signal mixing stage of the circuit provides input to a microprocessor which controls a single electronic variable resistor which adjusts the resistance between both the inverting and non-inverting inputs of a difference amplifier and ground. This form of non-periodic calibration, however, because it is only designed to calibrate during the initial ‘power-on’ sequence, presumes that the calibration parameters remain unchanged after initialization. Signal treatment by electronic components can change as the temperature changes. As an electronic device warms-up or as the ambient temperature changes, the initial calibration may be insufficient to provide the desired long term precision.